IBM/370 Assembler with command-level CICS, probably because I used it early in my career on an app where every program had to be small and fast. Nothing since then has approached the sheer fun of all that bit-diddling.
Same for me, except it was a 360 :) That, coffee, an 8 foot manual rack along with a copy of Stern's book on Assembly kept me going until the wee hours...
Actually my first small, fast program used only the 360 command set because that's what my tech school had taught me. It had lots of moves of variable length data, but my "mentor" at work didn't clue me in about MVCL so I rolled my own variable moves by modifying code in flight. Yikes. Before writing the 2nd program I read more code and more books and learned the right way to do it. Both programs are still actively maintained 25 years later, and that first gets a lot of WTF responses when a new guy lays eyes on it:-)